[Docs] Move documentation sources to docs/ and include CONTRIBUTING (#7584)

Rename doc/ to docs/ so GitHub picks up docs/CONTRIBUTING.md and Doxygen keeps including it, and update all references to the old layout:

- Doxygen output now goes to docs-output/, exposed by .gitignore
- docs workflow triggers on docs/** and publishes docs-output/html
- theme submodule moved to docs/doxygen/theme
- schema URLs in both XML parsers point at docs/xsd/
- README, developer docs, desktop workflow filters, .dockerignore and gcovr config updated for the new paths

The v3 card database schema is kept as docs/xsd/cards_v3.xsd.

docs/doxygen/pages/user_documentation/index.md

@@ -0,0 +1,30 @@
+@page user_reference User Reference
+
+## Deck Management
+
+- @subpage creating_decks
+- @subpage importing_decks
+- @subpage editing_decks
+- @subpage exporting_decks
+
+## Release Channels
+
+- @subpage beta_release
+
+## Settings
+
+- @subpage settings_folder
+
+## Card Pictures
+
+- @subpage custom_card_pictures
+
+## Troubleshooting
+
+- @subpage fixing_card_pictures
+- @subpage enabling_debug_logs
+
+## Syntax Help
+
+- @subpage search_syntax_help
+- @subpage deck_search_syntax_help